Re: [PATCH v3 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one

[David Hildenbrand]

On 12.01.21 12:27, Michal Hocko wrote:
> On Tue 12-01-21 12:11:21, David Hildenbrand wrote:
>> On 12.01.21 12:00, David Hildenbrand wrote:
>>> On 10.01.21 13:40, Muchun Song wrote:
>>>> If the refcount is one when it is migrated, it means that the page
>>>> was freed from under us. So we are done and do not need to migrate.
>>>>
>>>> This optimization is consistent with the regular pages, just like
>>>> unmap_and_move() does.
>>>>
>>>> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>>>> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>>> Acked-by: Yang Shi <shy828301@xxxxxxxxx>
>>>> ---
>>>>  mm/migrate.c | 6 ++++++
>>>>  1 file changed, 6 insertions(+)
>>>>
>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>> index 4385f2fb5d18..a6631c4eb6a6 100644
>>>> --- a/mm/migrate.c
>>>> +++ b/mm/migrate.c
>>>> @@ -1279,6 +1279,12 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>>>>  		return -ENOSYS;
>>>>  	}
>>>>  
>>>> +	if (page_count(hpage) == 1) {
>>>> +		/* page was freed from under us. So we are done. */
>>>> +		putback_active_hugepage(hpage);
>>>> +		return MIGRATEPAGE_SUCCESS;
>>>> +	}
>>>> +
>>>>  	new_hpage = get_new_page(hpage, private);
>>>>  	if (!new_hpage)
>>>>  		return -ENOMEM;
>>>>
>>>
>>> Question: What if called via alloc_contig_range() where we even want to
>>> "migrate" free pages, meaning, relocate it?
>>>
>>
>> To be more precise:
>>
>> a) We don't have dissolve_free_huge_pages() calls on the
>> alloc_contig_range() path. So we *need* migration IIUC.
>>
>> b) dissolve_free_huge_pages() will fail if going below the reservation.
>> In that case we really want to migrate free pages. This even applies to
>> memory offlining.
>>
>> Either I am missing something important or this patch is more dangerous
>> than it looks like.
> 
> This is an interesting point. But do we try to migrate hugetlb pages in
> alloc_contig_range? isolate_migratepages_block !PageLRU need to be

I didn't test it so far (especially in the context of virtio-mem or
CMA), but have a TODO item on my long list of things to look at in the
future.

> marked as PageMovable AFAICS. This would be quite easy to implement but
> a more fundamental question is whether we really want to mess with
> existing pools for alloc_contig_range.

Can these pages fall onto ZONE_MOVABLE or even MIGRATE_CMA? If yes, we
really want to. And I think both is the case for "ordinary" huge pages
allocated via the buddy.

> 
> Anyway you are quite right that this change has more side effects than
> it is easy to see while it doesn't really bring any major advantage
> other than the consistency.

Free hugetlbfs pages are special. E.g., they cannot simply be skipped
when offlining. So I don't think consistency actually really applies.


-- 
Thanks,

David / dhildenb